Objectives: To identify the frequency and types of microsatellite instability among a group of sporadic CRC patients and to correlate the findings with clinicopathological characteristics. Methods: During an 8-month period, all patients with sporadic CRC who attended to two teaching hospitals in Baghdad, Iraq were recruited to this cross-sectional study regardless of age, sex, ethnicity, or tumor characteristics. Demographic, clinical, and histopathological features were recorded. DNA was extracted from FFPE-blocks of the resected tumors and normal tissues. PCR amplification of five microsatellite mononucleotide repeat loci (BAT25, BAT26, NR-21, NR-24, and MONO-27) and 2 pentanucleotide repeat control markers (Penta C and Penta D) was performed to determine the MSI status. Capillary electrophoresis and Genetic Analyzer 3500 (Applied Biosystem, Japan) were used to separate and examine the products. Data were analyzed by Genescan software (Promega, USA). Instability of two or more loci is considered MSI-H. Result: In this study, ages of the 45 recruited patients ranged between 20-80 years, with a mean±SD of 55±12.3 years; of them, 31(68.9%) were ≥50 years; 25 (55.6%) were males. Rectal bleeding was the most frequent presenting feature [22 (48.9%)] patients; 23 (51.1%) of CRCs were located at recto-sigmoid region, 29 (64.4%) were T3 tumors, 34(75.5%) were non-mucinous adenocarcinoma, 39(86.7%) were moderately differentiated, 17 (37.8%) patients had stage III tumors; and 25 (55.5%) had lymphovascular invasion. MSI-H was seen in 5/45 (11.1%) patients; 3(60%) of them were ≥50 years, 4(80%) were males, 3(60%) were smokers, 2 (40%) presented with intestinal obstruction and altered bowel habits each; 4(80%) had T3 tumors, 3(60%) had mucinous adenocarcinomas [p=0.004], 2(40%) had stage II tumor and stage III each. Conclusion: The frequency of MSI-H among the recruited patients with CRC was 5/45 (11.1%) and it was significantly associated with mucinous adenocarcinoma subtype. NR-24 and NR-21 were the most prevalent instable markers.
